As well known in the art, optical fiber technology has enabled realization of imaging systems known as industrial endoscopes (or borescopes) which are widely used in various industrial applications. Such imaging systems provide means for inspecting such internal structures enclosed within, for example, an industrial machine, that are otherwise inaccessible and can not be inspected with direct line of sight imaging modalities without first painstakingly dismantling the industrial machine.
Such imaging systems are essentially based on using a fiber bundle as an image conduit. At a proximal end of the image conduit, an optical arrangement focuses light gathered from a region of interest onto a plane of optical fiber bundle. The fiber bundle transmits the light from the proximal end to a distal end in the form of individual pixels, one pixel per optical fiber included in the fiber bundle. At the distal end of the image conduit, another optical arrangement focuses the light onto an image plane. A set of light sensing elements, such as a charge-coupled device (CCD) array, a complementary metal-oxide semiconductor (CMOS) array, a photographic film, and so on, are arranged along the image plane to sense the light received at the image plane such that an image is accordingly generated. In a typical use case, the proximal end of the image conduit is inserted through a small opening to reach to an otherwise inaccessible area within an interior of a machine or a component thereof. Such imaging systems are generally known in the prior art such as those disclosed in U.S. Pat. No. 5,986,752 issued to Morito et al, U.S. Pat. No. 4,849,626 issued to Franklin et al, U.S. Pat. No. 4,281,929 issued to Lord et al, and so on.
Although such imaging systems are widely available in general and satisfactorily address several desired applications, their use in certain applications poses specific challenges. One such application is online condition monitoring in turbine engines.
In a typical turbine engine, also known as a gas turbine or a combustion turbine, an upstream rotating compressor is coupled to a downstream turbine, and a combustion chamber is located in-between. A gas stream enters the turbine engine from the compressor end and is highly pressurized in the upstream compressor; the compressed gas stream subsequently enters the combustion chamber at a high velocity, fuel is added thereto and ignited to impart additional energy to the gas stream; the energized gas stream subsequently drives the downstream turbine.
Such turbine engines operate at very high temperatures which may exceed 1,200 degrees Centigrade. Moreover, the gas stream propagates through the turbine engine at extremely high velocities and also, the turbine engine experiences strong mechanical vibrations during operation leading to high mechanical stress. Accordingly, an imaging system desired to be used for online condition monitoring in a gas turbine must be designed to withstand such high-stress environment that is encountered therein.
In recent years, intensive research and development work has been conducted to design imaging systems suitable for online condition monitoring in turbine engines and other such harsh environments; and various systems and methods towards this end have been proposed.
One such system and method is known from U.S. Pat. No. 7,486,864 issued to Diatzikis and assigned to Siemens Energy, Inc. The aforementioned patent discloses an imaging system including at least one photonic crystal fiber having an imaging end and a processing end; an imaging camera operably connected to the processing end of the at least one photonic crystal fiber; and an imaging processor operably connected to the imaging camera. The photonic crystal fiber can comprise a sapphire cladding and defines a hollow core. The imaging end of the at least one photonic crystal fiber can capture light in the area of interest and guides the light to the imaging camera. The processor can generate an image based on the light.
Another such system is known from U.S. Pat. No. 8,184,151 issued to Zombo et al and assigned to Siemens Energy, Inc. The aforementioned patent discloses an imaging system for imaging an internal component within a gas turbine engine. The imaging system includes a flexible imaging bundle comprising an imaging end for imaging a component in a hot gas path of the engine, and a viewing end providing an image of the component at a location displaced from the hot gas path. The imaging end comprises a plurality of receptor sites defining an imaging plane for receiving an image of the component. The flexible imaging bundle defines a plurality of separate light paths defined by a plurality of high temperature optical elements, each light path corresponding to a receptor site. The viewing end comprises a plurality of emission sites, each emission site corresponding to a receptor site wherein a location and number of the emission sites forming the viewing end have a one-to-one correspondence to the location and number of the imaging sites forming the imaging end to effect transmission of a coherent image through the flexible imaging bundle from the imaging end to the viewing end.
Despite significant advancements, various state of the art industrial endoscopes suffer from several disadvantages with regard to condition monitoring in a turbine engine.
In current imaging systems, individual optical fibers are mutually optically isolated; each optical fiber transmits light corresponding to an individual pixel in the acquired image. Therefore, in order to obtain an image with a satisfactory resolution level, a fiber bundle with relatively very large number of optical fibers has to be used. This, in turn, leads to an undesirable total thickness of an image conduit used in such imaging systems. As a result, several desired regions of interest within a turbine engine, such as regions located inside a vane, still remain inaccessible.
The problem of unsuitable form-factor is further aggravated in case of imaging systems designed for online condition monitoring in turbine engines due to additional measures required to withstand high temperature and other harsh ambient conditions. In particular, fiber bundles are formed with special coatings on individual fibers for robustness, which undesirably prevent high density packing of the optical fibers. If it is desirable to keep the thickness of fiber bundle small enough (e.g. less than 8 mm) such as to fit through constricted regions such as the inside a vane insert, the number of optical fibers that may be included in the fiber bundle are limited to a few hundreds. The limitation of the total number of fibers to a few hundreds restricts any type of imaging applications to very low resolution.
In light of the foregoing, there is a need for an improved system and method for image acquisition suitable for use in a turbine engine. The improved system and method should ensure a compact and thin form-factor of the system and in particular, an image conduit therein, such that accessibility of various constricted regions within a turbine engine is improved. At the same time, the improved system and method should facilitate image acquisition with a satisfactorily high resolution level.